It is the goal of most burner applications to attain, among others, the most effective combustion possible in order to achieve the maximum degree of efficiency. However, the temperature limit is usually restricted by the temperature resistance of the available burner material.
Temperatures higher than permitted by the available materials occur in particular with strong exothermal chemical reaction, such as during the synthesis of intermediate or end products. Such chemical reactions are frequently performed with freely burning flame, which burns far remote from the combustion material.
In addition, with these exothermal chemical reactions there exist, depending upon the properties of the employed materials, specifically designed reactors comprising one or several burner units and a combustion chamber. Since, however, the employed materials and the products frequently present highly toxic and corrosive properties, such reactors must be designed according to particularly strict safety requirements, which constitutes significant limitation with respect to the selection of material.
The following description deals mainly with synthesis of hydrochloric acid, in which chlorine and hydrogen are reacted with each other. If mention is frequently made is this text of the synthesis of hydrochloric acid, it is not intended to be limitative but only exemplary. The represented principles can be transferred to all similar reactions, in particular such reactions where high temperatures may develop.
With the synthesis of hydrochloric acid, the reaction chamber, in principle, is a diffusion flame, which is located in an expanded combustion chamber. Relatively simple devices are used as burners which are made of corrosion-resistant materials and comprise an inner and an outer pipe. The chlorine-containing gases are supplied via the inner pipe, which frequently presents a specifically designed burner head. The hydrogen-containing gases, however, are introduced through a free annular gap between the pipes, so that the hydrogen practically envelopes the chlorine-containing gases. The two gases only come into contact directly after exit from the burner, they react with each other and form a long flame which expands in the combustion chamber.
Thorough mixing of the reactants in the flame takes mainly place by diffusion. Because of the high combustion temperatures, the walls of the combustion chamber need to be away as far as possible. In addition, these walls are cooled in most instances.
This, however, produces a great disadvantage with this type of combustion, because the outer regions of such flame cool down to such extent from heat transfer to the cooled walls, that the conversion of the reactants takes place only slowly or comes to a stop. Under certain circumstances this results in unreacted chlorine within the formed hydrochloric acid during exit of the hot combustion gases from the combustion chamber. The percentage of the unreacted chlorine is therefore reduced by providing a hydrogen excess ranging between 5 and 10%.
It is desirable with this type of applications, in particular with synthesis of hydrochloric acid, to attain higher flame stability. The usual physical parameters for adjustment, however, are highly restricted. With under-pressure, for example, the stability of the flame declines,—its expansion clearly increases. Combustion at higher pressures, however, is not customary practice up to now. Also, specifically designed pre-mix burners cannot be employed due to the danger of flame backfire without self-ignition of the mixture of chlorine and hydrogen.
Another topic to be addressed here is the disposal of noxious gas. A customary specific embodiment of a chemical reactor for disposal of noxious matter is a hot combustion chamber lined with refractory materials, in which the combustion gases are brought to reaction with traditional burners. The noxious gases to be disposed of are passed into a combustion chamber with the aid of a separate supply line. The combustion chamber is dimensioned in such manner that a certain minimum dwell time exists with high temperature for final combustion of reaction gases. Such devices are preferably employed for after-combustion of chlorine-containing compounds or exhaust gases. The minimum dwell time generally determines the size of such combustion chambers. It is true, however, that with better mixing and more intensive heat, the size can be reduced.
The above cited discourse demonstrates that such fixtures can only be manufactured with maintenance of a certain minimum size, in other words, the fixtures usually occupy a great deal of space. For example, in the catalog of Messrs. SGL-TECHNIK GmbH. Apparatebau, Werner von Siemens Straβe 18, DE-86405 Meitingen, an HCL-Synthesis Oven is shown on page 3, from which it is apparent from the size of persons depicted in the same figure that its approximate size is over 10 m long. Said length should be able to be drastically reduced.